This invention relates to catalytic combusters for burning gases supplied thereto, and more particularly to a combuster having a unique mixing section for intimately mixing the gases before supplying them to the combustion catalyst.
Catalytic combusters (a.k.a. catalytic burners) for burning gases are known and generally comprise a gas inlet section, a gas mixing section, and a catalyst, downstream of the mixing section, where combustion occurs. Combusters are used with a variety of gaseous, hydrogenous fuels, and have many applications one of which is to heat the fuel processor, or other components, of a fuel cell system. Fuel-cell-system combusters burn the tailgases (effluents) from the anode and cathode compartments of the fuel cell, which tailgases comprise hydrogen and oxygen respectively. So-called xe2x80x9cradial inletxe2x80x9d, prior art combusters have a mixing section like that shown in FIG. 1 which tend to (1) have a high pressure drop, and (2) produce a slow moving layer of well-mixed gases adjacent the perforated perimeter wail that defines the mixing chamber, (3) a more rapidly moving central region of poorly mixed gases, and (4) a relatively long residence time for the gases in the mixing section. Because of the long residence time in the mixing section, the heat that is generated in the combustion section, downstream of the mixing section, can be transmitted back into the mixing section where it can cause auto-ignition of the well-mixed gases (i.e. adjacent the perforated wall). This, in turn, can cause overheating or localized heating that can be detrimental to the combuster.
The present invention is an improved catalytic combuster that has a mixing section upstream of the combustion section that quickly and thoroughly mixes the gases before they enter the catalytic combustion section. The improved combuster has an easily maintainable design that is readily assembleable/disassembleable, has a relatively low pressure drop, and has a low residence time (and hence reduced auto-ignition potential) for the gases in the mixing section, yet still provides excellent mixing of the gases in the mixing section. More specifically, the invention is directed to an improvement to the mixing section of the combuster that achieves intimate mixing of the gases throughout the mixing section in a short distance from the inlet end of the combuster. The improved mixing section comprises a housing having a wall that defines the mixing chamber, a first gas-permeable, upstream distributor (e.g. open cell foam, sintered metal or a perforated plate) at the entrance to the mixing chamber for admitting one of the gases into the mixing chamber in a first direction, and a a second gas distributor in the form of a multi-port injector extending into the mixing chamber and surrounded by the housing wall for admitting the other gas into the mixing chamber in a plurality of directions generally perpendicular to the first direction
According to one embodiment of the invention, the multi-port injector comprises a distribution ring having a first, inner, ported (i.e. perforated), annular wall that defines a hole in the center of the distribution ring, and a second, outer, ported annular wall spaced radially outboard from the first ported wall. The inner and outer walls together define an annular cavity that receives the second gas and is adapted to inject the second gas into the mixing chamber in a radially inwardly second direction through the first permeable wall, and a radially outwardly third direction through the second perforated wall. At least one conduit communicates the inlet with the distribution ring. Preferably, multiple such conduits extend between the inlet and the ring. Each of the conduits has a mouth that receives the other gas from the inlet. The several mouths are arranged in a circle concentric with the hole in the distribution ring, with the mouths opening through a baffle plate that separates the inlet from the vestibule. A conical deflector, concentric with, and radially inboard of, the circle of mouths, directs the second gas from the inlet into the conduits that supply the distribution ring.
According to another embodiment of the invention, the injector comprises a plurality of blind gas distribution tubes, each defined by a sidewall and extending into the mixing chamber downstream of the first gas distributor. The sidewalls each have a plurality of ports therein downstream of the first gas distributor for injecting the other gas into the mixing chamber in a direction generally perpendicular to the direction of flow of the first gas.
An inlet section of the combuster supplies the other gas to the injector. A gas-permeable, homogenizing diffuser (preferably an open-cell foam) at the downstream end of the mixing chamber restricts the outflow of the gases from the mixing chamber to promote mixing upstream of the diffuser and to distribute the outflow substantially uniformly over the cross section of the combuster transverse the length of the combuster at the entrance to the combustion section.
According to significant aspect of the invention, a plurality of openings are provided in the housing upstream of the distributor for supplying a first gas behind (i.e. upstream) the distributor. An annular plenum surrounds the openings (and preferably the downstream diffuser), and serves to supply the one gas to the openings in the housing behind the gas distributor. The gas flows in the plenum in a direction that is countercurrent to the direction of flow of other gas through the mixing section which helps to cool the mixing section and further suppress the possibility of auto-ignition. The combuster includes a vestibule located between the openings in the housing and the gas distributor to receive the one gas from the openings
According to a preferred embodiment of the invention, the several sections (i.e. inlet, mixing and combustion) of the combuster are each separate and discrete units that are connected to the next adjacent unit by means of a quick-disconnect connection to provide convenient access to the innards of the combuster for readily maintaining the combuster or modifying it (e.g. during design development).
Combusters made in accordance with the distributor ring embodiment of the invetion have demonstrated 80+% mixing of the gases in 70% of the cross sectional area (i.e. transverse the direction of flow through the combuster) of the mixing section. In contrast only about 45% of the cross sectional area of the radial mixers (see FIG. 1) contained 80+% mixed gases. Such improved mixing is achieved in combusters in accordance with the present invention that have demonstrated as much as 34% lower pressure drop, and shorter residence times than the prior art radial inlet combusters.